A hands-free communication apparatus has been proposed, for example, in JP-A-2005-198093 or US 2005/0286714 corresponding to JP-A-2006-33802. Such a hands-free communication apparatus has an echo canceller that performs an echo cancellation calculation on outgoing and incoming voice signals to reduce an acoustic echo signal contained in the outgoing voice signal. The acoustic echo originates from the fact that a part of the incoming voice signal outputted from a speaker is picked up by a microphone and transmitted back to a calling party as a part of the outgoing voice signal.
In a case where such a hands-free communication apparatus is used in a vehicle, an incoming voice signal from a calling party is amplified by an on-board power amplifier originally installed on the vehicle and then outputted from an on-board vehicle speaker originally installed on the vehicle. When a gain set value of the power amplifier is set to less than a gain threshold value, the power amplifier lineally amplifies the incoming voice signal according to the gain set value. In contrast, when the gain set value of the power amplifier is set equal to or greater than the gain threshold value, the power amplifier nonlinearly amplifies the incoming voice signal.
When the user cannot clearly hear the incoming voice signal (i.e., calling party's voice) outputted from the speaker due to surrounding noise, the user increases a volume set value of a volume switch to increase a volume of the speaker. In this case, the gain set value of the power amplifier increases with the volume set value of the volume switch and may become equal to or greater than the gain threshold value. As a result, although the user can easily hear the incoming voice signal, the incoming voice signal may be distorted due to the fact that the incoming voice signal is amplified with high gain. Due to the distortion, power of an incoming voice signal that is outputted from the speaker, picked up by a microphone, and transmitted back to the calling party as a part of an outgoing voice signal is different from power of an incoming voice signal transmitted from the calling party. Therefore, an echo canceller cannot suitably reduce an acrostic echo signal contained in the outgoing voice signal.